1. Field of the Invention
The present invention relates to well logging tools and methods, and more particularly to methods for analyzing extracted formation fluids by nuclear magnetic resonance (NMR) techniques.
2. Background Information
Downhole formation fluid sampling tools, such as the Schlumberger Modular Formation Dynamics Tester (MDT), withdraw samples of fluids from earth formations for subsequent analyses. These analyses are needed to characterize physical properties such as water and oil volume fractions, oil viscosity, and water salinity, among others. This knowledge is needed to interpret wireline well logs, and to plan for the efficient exploitation of the reservoir.
In an undisturbed reservoir, formation fluids sometimes partially support the overburden pressure of the earth. When a fluid-bearing formation is penetrated by drilling, formation fluids will flow into the borehole if it is at a lower pressure. The uncontrolled escape of combustible hydrocarbons to the surface (xe2x80x9cblowoutxe2x80x9d) is extremely dangerous, so oil wells typically are drilled under pressure. During drilling, fluid (xe2x80x9cmudxe2x80x9d) is circulated through the well to carry rock chips to the surface. The mud is densified with heavy minerals such as barite (barium sulfate, 4.5 g/cm3) to ensure that borehole pressure is higher than formation pressure. Consequently, fluid is forced into the formation from the borehole (xe2x80x9cinvasionxe2x80x9d). Usually particles are prevented from entering the formation by the filtering action of the porous rock. Indeed, the filtration process is self-limiting because solids, purposely mixed in the drilling fluid, form a filter cake (xe2x80x9cmud cakexe2x80x9d) at the surface of the borehole. Nonetheless liquid (xe2x80x9cmud filtratexe2x80x9d) can penetrate quite deeplyxe2x80x94as much as several meters into the formation. The filtrate can be either water with various soluble ions, or oil, depending on the type of mud used by the driller. Therefore, the fluid samples withdrawn are mixtures of native formation fluids (including gas, oil and/or water) and the filtrate of mud that was used to drill the well.
Sample contamination of formation fluids by mud filtrate is universally regarded as the most serious problem associated with downhole fluid sampling. It is essential that formation fluid, not mud filtrate, is collected in the sample chambers of the tool. Therefore fluid from the formation is pumped through the tool and into the borehole until it is believed contamination has been reduced to an acceptable level. Thus it is necessary to detect mud filtrate in the fluid sample, to decide when to stop pumping the fluid through the tool and to start collecting it for analysis.
Several measurements are routinely made in fluid sampling tools to detect mud filtrate contamination:
Resistivity indicates the presence of water. The measurement uses the low frequency electrode technique. Unless there is a continuous conducting path between the electrodes, there is no sensitivity to the presence of water. Even with a conducting path, the method is unable to separate the effects of water volume, salinity, and flow geometry. The measurement is simple and often useful, but inherently non-quantitative.
Dielectric constant can distinguish oil from water, but not one oil from another. Moreover the dielectric constant measurement depends on the flow regime of oil/water mixtures.
Flow line pressure and temperature provide no information on fluid properties.
Optical Fluid Analyzers (e.g. Schlumberger OFA) can detect contamination in many cases. It is particularly effective when the mud filtrate is aqueous and the flowing formation fluid is pure hydrocarbon, since there is a large contrast between water and oil in the near infrared band. However, it does less well when the filtrate is oil based, or when the formation fluid is a mixture of oil and water.
Thus, no presently deployed system is generally useful for determining the contamination level of sampled formation fluids. There is a clear need for an apparatus and method which monitors contamination while the sample is being taken, and indicates when contamination has been reduced to an acceptably low level.
Downhole formation fluid sampling tools can withdraw samples of fluids from earth formations and transport them to the surface. The samples are sent to fluid analysis laboratories for analysis of composition and physical properties. There are many inefficiencies inherent in this process.
Only about six samples can be collected on each descent (xe2x80x9ctripxe2x80x9d) of the tool into the borehole. Because fluid sampling tools are deployed from drilling rigs, and because the rental charge for such rigs can exceed $150,000 per day in the areas where fluid sampling is most often conducted, economic considerations usually preclude multiple trips in the hole. Thus, oil producing formations are almost always undersampled.
The samples undergo reversible and irreversible changes as a result of the temperature and/or pressure changes while being brought to the surface, and as a result of the transportation process. For example, gases come out of solution, waxes precipitate, and asphaltenes chemically recombine. Irreversible changes eliminate the possibility of ever determining actual in situ fluid properties. Reversible changes are deleterious because they occur slowly and therefore impact sample handling and measurement efficiency.
The transportation and handling of fluids uphole entails all the dangers associated with the handling of volatile and flammable fluids at high pressure and temperature. After analyses are complete, the samples must be disposed of in an environmentally acceptable manner, with associated financial and regulatory burdens.
Because fluid analysis laboratories are frequently distant from the well site, there are substantial delaysxe2x80x94often several weeksxe2x80x94in obtaining results. If a sample is for some reason corrupted or lost during sampling, transportation, or measurement, there is no possibility of returning to the well to replace it.
Thus there is a clear need for immediate analysis of fluid samples at formation temperature and pressure within the downhole sampling tool.
Nuclear magnetic resonance (NMR) can be used to monitor contamination and analyze fluid samples in fluid sampling tools under downhole conditions. Measurements are performed in the flow line itself. The methods are inherently noninvasive and noncontacting. Since magnetic resonance measurements are volumetric averages, they are insensitive to flow regime, bubble size, and identity of the continuous phase. Nuclear magnetic resonance of hydrogen nuclei (protons) is preferred because of the ubiquity and good NMR characteristics of this nuclear species. However, magnetic resonance of other nuclear species is useful and so included within the scope of the present invention.
In general, the methods of analyzing a fluid according to the invention include introducing a fluid sampling tool into a well bore that traverses an earth formation. The fluid sampling tool extracts the fluid from the earth formation into a flow channel within the tool. While the fluid is in the flow channel, a static magnetic field is applied, and an oscillating magnetic field applied. Magnetic resonance signals are detected from the fluid and analyzed to extract information about the fluid.
These and other features of the invention are described in more detail in figures and in the description below.